This device has been used to study organic particles (polystyrene, etc.) and biological particles (red blood cells) and, particularly in the case of the latter, their state of aggregation. The device and the apparatus described above make it possible to study, under advantageous conditions, the properties of particles (size, density, compaction, aggregation) as well as the state of the sediment formed. The rheological properties of the blood are largely governed by those of the red blood cells, on account of their large concentration, and they depend upon the deformability and aggregatability of these particles. Consequently, it is important in hospital practice to determine accurately the state of aggregation, or the agglutination of these cells. To evaluate the aggregation of the red blood cells with one another, usually the rate of sedimentation of the blood in the plasma is measured. The rate of sedimentation depends on the density of the red blood cells and the plasma, depends on the viscosity of the plasma and depends on the size of the red blood as well as of the aggregates. One classical method consists in measuring the height of a column of plasma above the plasma-blood interface after the first and second hour of sedimentation. It is also possible, on the basis of a curve of sedimentation obtained as a function of time, to determine the maximum rate of sedimentation of the red blood cells by measuring the greatest slope of the tangent to the sedimentation curve.
Ultrasonic methods can also be used. These are based on interactions, particularly the diffusion of the ultrasonic waves, with the substance concerned. It is known, indeed, that the intensity of an ultrasonic beam that propagates through a medium decreases as a function of the distance travelled. This attenuation of intensity of the ultrasonic beam is derived from a) the reflection occurring on the surface of elements constituting the medium and whose size is greater than the wavelength of the ultrasonic wave, from b) the diffusion of the beam which occurs when the size of the particles in suspension in the medium is less than or equal to the wavelength of the ultrasonic wave used, or from c) the absorption of ultrasounds which depends on the thickness and on the nature of the medium the ultrasonic wave travelled through. The interaction between the ultrasonic waves and the blood at the frequencies habitually used in medical diagnosis always gives rise to diffused waves. Analysis of the amplitude and frequency of the diffused waves yields information on the size, the nature and the movement of this obstacle. Experiments analyzing the amplitude of ultrasonic waves have been conducted in this connection and it has been possible to measure the variations in the back scattering coefficient of ultrasonic waves for blood suspensions as a function of the sedimentation rate of the red blood cells and it has been possible to identify a difference between non-aggregated and aggregated red blood cells.